JP7380841B2 - secondary battery - Google Patents

Description

The present technology relates to a secondary battery.

BACKGROUND OF THE INVENTION As various electronic devices such as mobile phones have become widespread, secondary batteries are being developed as a power source that is small and lightweight and provides high energy density. This secondary battery includes an electrolyte as well as a positive electrode and a negative electrode, and various studies have been made regarding the configuration of the secondary battery.

Specifically, in order to improve cycle characteristics, fluorinated cyclic carbonates (cyclic carbonates with halogen atoms) and unsaturated cyclic carbonates (cyclic carbonates with unsaturated bonds) are included in the electrolyte as additives. (For example, see Patent Document 1.) In addition, sulfur-containing cyclic compounds (such as 1,3-propane sultone) and dinitrile compounds (compounds represented by NC-R-CN) are also used as additives for the electrolyte (for example, Patent Document 2, (See 3). In these cases, the content of each additive is specified, and the total sum of these contents is also specified (see, for example, Patent Documents 4 to 6).

Japanese Patent Application Publication No. 2016-143449 JP2016-048624A JP2013-065540A Japanese Patent Application Publication No. 2011-192632 JP2006-012806A Japanese Patent Application Publication No. 2004-342585

Although various studies have been made regarding the performance of secondary batteries, the electrical resistance characteristics of the secondary batteries are still insufficient, so there is room for improvement.

The present technology has been developed in view of such problems, and its purpose is to provide a secondary battery that can obtain excellent electrical resistance characteristics.

A secondary battery according to an embodiment of the present technology includes a positive electrode, a negative electrode in which a film containing an organic substance is formed on the surface of a negative electrode active material layer, a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, It is equipped with an electrolytic solution containing a multi-nitrile chain compound.

Mass spectrometry of the electrolyte using gas chromatography-mass spectrometry (GC/MS) reveals that sulfur-containing cyclic compounds, fluorinated cyclic carbonates, unsaturated cyclic carbonates, and The content of each polynitrile chain compound satisfies the following relationship: content of polynitrile chain compound>content of fluorinated cyclic carbonate>content of sulfur-containing cyclic compound>content of unsaturated cyclic carbonate In addition, the total of the content of the sulfur-containing cyclic compound, the content of the fluorinated cyclic carbonate, the content of the unsaturated cyclic carbonate, and the content of the polynitrile chain compound is 5.0% by weight or more11. It is 0% by weight or less.

Elemental analysis of the film in a range from the surface to a depth of 20 nm using X-ray photoelectron spectroscopy (XPS) reveals that the elemental concentration of carbon is 80 atomic % or more and 94 atomic % or less.

The above-mentioned "negative electrode in which a film containing an organic substance is formed on the surface of the negative electrode active material layer" means that the secondary battery has already been charged and discharged for one or more cycles after assembly, that is, for the secondary battery after assembly. This means that the stabilization process described later has already been applied. This stabilization treatment is a charge/discharge treatment for forming a film containing an organic substance on the surface of the negative electrode active material layer. Note that details of the stabilization process will be described later.

"Sulfur-containing cyclic compound" is a general term for cyclic compounds containing one or more sulfur as a constituent element. "Fluorinated cyclic carbonate ester" is a general term for cyclic carbonate esters containing one or more fluorine as a constituent element. "Unsaturated cyclic carbonate ester" is a general term for cyclic carbonate esters containing one or more unsaturated bonds (carbon-carbon double bonds). "Multi-nitrile chain compound" is a general term for chain compounds containing two or more cyano groups. The details of each of the sulfur-containing cyclic compound, fluorinated cyclic carbonate, unsaturated cyclic carbonate, and multinitrile chain compound will be described later.

According to the secondary battery of one embodiment of the present technology, the electrolyte contains a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, and a polynitrile chain compound, and the film containing an organic substance is a negative electrode. Since it is formed on the surface of the active material layer and satisfies a series of conditions regarding mass spectrometry of the electrolyte using GC/MS and elemental analysis of the film using XPS, excellent electrical resistance properties are obtained. be able to.

Note that the effects of the present technology are not necessarily limited to the effects described here, and may be any of a series of effects related to the present technology described later.

FIG. 1 is a perspective view showing the configuration of a secondary battery in an embodiment of the present technology. FIG. 2 is a cross-sectional view showing the configuration of the battery element shown in FIG. 1. FIG. FIG. 2 is a block diagram showing the configuration of an application example of a secondary battery.

Hereinafter, one embodiment of the present technology will be described in detail with reference to the drawings. The order of explanation is as follows.

1. Secondary battery 1-1. Configuration 1-2. Physical properties 1-3. Operation 1-4. Manufacturing method 1-5. Action and effect 2. Modification example 3. Applications of secondary batteries

<1. Secondary battery>
First, a secondary battery according to an embodiment of the present technology will be described.

The secondary battery described here is a secondary battery whose battery capacity is obtained by utilizing intercalation and desorption of electrode reactants, and includes a positive electrode, a negative electrode, and an electrolytic solution that is a liquid electrolyte. In this secondary battery, the charging capacity of the negative electrode is larger than the discharge capacity of the positive electrode in order to prevent electrode reactants from depositing on the surface of the negative electrode during charging. That is, the electrochemical capacity per unit area of the negative electrode is set to be larger than the electrochemical capacity per unit area of the positive electrode.

The type of electrode reactant is not particularly limited, but specifically light metals such as alkali metals and alkaline earth metals. Specific examples of alkali metals include lithium, sodium, and potassium, and specific examples of alkaline earth metals include beryllium, magnesium, and calcium.

In the following, a case where the electrode reactant is lithium will be exemplified. A secondary battery whose battery capacity is obtained by utilizing intercalation and desorption of lithium is a so-called lithium ion secondary battery. In this lithium ion secondary battery, lithium is intercalated and released in an ionic state.

<1-1. Configuration>
FIG. 1 shows a perspective configuration of a secondary battery, and FIG. 2 shows a cross-sectional configuration of the battery element 10 shown in FIG. 1. However, in FIG. 1, the battery element 10 and the exterior film 20 are shown separated from each other, and in FIG. 2, only a part of the battery element 10 is shown.

As shown in FIG. 1, this secondary battery includes a battery element 10, an exterior film 20, a positive electrode lead 14, and a negative electrode lead 15. The secondary battery described here is a laminate film type secondary battery that uses a flexible (or pliable) exterior member (exterior film 20) as an exterior member for housing the battery element 10. .

[Exterior film]
As shown in FIG. 1, the exterior film 20 is a single film-like member, and is foldable in the direction of arrow R (dotted chain line). As described above, since this exterior film 20 houses the battery element 10, it also houses an electrolyte together with the positive electrode 11 and the negative electrode 12, which will be described later. Note that the exterior film 20 is provided with a recessed portion 20U (so-called deep drawing portion) for accommodating the battery element 10.

Specifically, the exterior film 20 is a three-layer laminate film in which a fusing layer, a metal layer, and a surface protection layer are laminated in this order from the inside, and when the exterior film 20 is folded, they face each other. The outer peripheral edges of the fusion layers are fused to each other. The adhesive layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protective layer contains a polymer compound such as nylon.

However, the structure (number of layers) of the exterior film 20 is not particularly limited, and may be one or two layers, or four or more layers.

An adhesive film 21 is inserted between the exterior film 20 and the positive electrode lead 14, and an adhesive film 22 is inserted between the exterior film 20 and the negative electrode lead 15. Each of the adhesive films 21 and 22 is a member that prevents outside air from entering into the interior of the exterior film 20, and is made of a polymer compound such as polyolefin that has adhesiveness to each of the positive electrode lead 14 and the negative electrode lead 15. Contains one or more of these types. The polyolefins include polyethylene, polypropylene, modified polyethylene, and modified polypropylene. However, one or both of the adhesive films 21 and 22 may be omitted.

[Battery element]
As shown in FIGS. 1 and 2, the battery element 10 is housed inside an exterior film 20, and includes a positive electrode 11, a negative electrode 12, a separator 13, and an electrolyte (not shown).

In this battery element 10, a positive electrode 11 and a negative electrode 12 are stacked on each other with a separator 13 in between, and the positive electrode 11, negative electrode 12, and separator 13 are arranged around a winding axis (a virtual axis extending in the Y-axis direction). It is a wound structure (wound electrode body). Thereby, the positive electrode 11 and the negative electrode 12 are opposed to each other with the separator 13 in between.

Here, the three-dimensional shape of the battery element 10 is a flat shape. That is, the shape of the cross section of the battery element 10 that intersects the winding axis (the cross section along the It is. The long axis is a virtual axis that extends in the X-axis direction and has a relatively large length, and the short axis extends in the Z-axis direction that intersects the X-axis direction and has a relatively small length. It is a virtual axis with a

(positive electrode)
As shown in FIG. 2, the positive electrode 11 includes a positive electrode current collector 11A having a pair of surfaces, and two positive electrode active material layers 11B disposed on both sides of the positive electrode current collector 11A. However, the positive electrode active material layer 11B may be arranged only on one side of the positive electrode current collector 11A.

The positive electrode current collector 11A includes one or more types of conductive materials such as metal materials, and the metal materials include aluminum, nickel, stainless steel, and the like.

The positive electrode active material layer 11B includes one or more types of positive electrode active materials capable of intercalating and deintercalating lithium, and may further include a positive electrode binder, a positive electrode conductive agent, and the like. .

The type of positive electrode active material is not particularly limited, but specifically, it is a lithium-containing compound such as a lithium transition metal compound. This lithium transition metal compound is a compound containing lithium and one or more types of transition metal elements as constituent elements, and may further contain one or more types of other elements. The types of other elements are not particularly limited, but specifically, they are elements belonging to Groups 2 to 15 of the long period periodic table (excluding transition metal elements). The type of lithium transition metal compound is not particularly limited, but specific examples include oxides, phosphoric acid compounds, silicate compounds, and boric acid compounds.

Specific examples of oxides include LiNiO ₂ , LiCoO ₂ , LiCo _0.98 Al _0.01 Mg _0.01 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , LiNi _{0.3 3} Co _0.33 Mn _0.33 O ₂ , Li _1.2 Mn _0.52 Co _0.175 Ni _0.1 O ₂ , Li _1.15 (Mn _0.65 Ni _0.22 Co _0.13 )O ₂ and LiMn ₂ O ₄ . Specific examples of phosphoric acid compounds include LiFePO ₄ , LiMnPO ₄ , LiFe _0.5 Mn _0.5 PO ₄ and LiFe _0.3 Mn _0.7 PO ₄ .

The positive electrode binder contains one or more of synthetic rubber, polymer compounds, and the like. Synthetic rubbers include styrene-butadiene rubber, fluorine-based rubber, and ethylene propylene diene. High molecular compounds include polyvinylidene fluoride, polyimide, and carboxymethylcellulose.

The positive electrode conductive agent contains one or more types of conductive materials such as carbon materials, and the carbon materials include graphite, carbon black, acetylene black, and Ketjen black. However, the conductive material may be a metal material, a polymer compound, or the like.

The method for forming the positive electrode active material layer 11B is not particularly limited, but specifically, it may be one or more of coating methods.

(Negative electrode)
As shown in FIG. 2, the negative electrode 12 includes a negative electrode current collector 12A having a pair of surfaces, and two negative electrode active material layers 12B and two coatings 12C disposed on both sides of the negative electrode current collector 12A. Contains. However, the negative electrode active material layer 12B and the coating 12C may be arranged only on one side of the negative electrode current collector 12A.

The negative electrode current collector 12A includes one or more types of conductive materials such as metal materials, and the metal materials include copper, aluminum, nickel, stainless steel, and the like.

The negative electrode active material layer 12B contains one or more types of negative electrode active materials capable of intercalating and deintercalating lithium, and may further contain a negative electrode binder, a negative electrode conductive agent, and the like. . The details regarding the negative electrode binder are the same as the details regarding the positive electrode binder, and the details regarding the negative electrode conductive agent are the same as the details regarding the positive electrode conductive agent.

The type of negative electrode active material is not particularly limited, but specifically includes carbon materials, metal materials, and the like. Carbon materials include graphitizable carbon, non-graphitizable carbon, and graphite, and examples of the graphite include natural graphite and artificial graphite. A metal-based material is a material that contains as a constituent element one or more of metal elements and metalloid elements that can form an alloy with lithium, and the metal elements and metalloid elements include silicon and metalloid elements. such as tin. However, the metallic material may be a single substance, an alloy, a compound, a mixture of two or more thereof, or a material containing phases of two or more thereof.

Specific examples of metallic materials include _SiB4 , _SiB6 , _Mg2Si , Ni2Si, _TiSi2 , _MoSi2 , _CoSi2 , _NiSi2 , _{CaSi2 , CrSi2} , _Cu5Si , _FeSi2 , _MnSi2 , NbSi ₂ , TaSi ₂ , VSi ₂ , WSi ₂ , ZnSi ₂ , SiC, Si ₃ N ₄ , Si ₂ N ₂ O, SiO _v (0<v≦2), LiSiO, SnO _w (0<w≦2), These include SnSiO ₃ , LiSnO and Mg ₂ Sn. However, v of SiO _v may satisfy 0.2<v<1.4.

The method of forming the negative electrode active material layer 12B is not particularly limited, but specifically, any one of a coating method, a gas phase method, a liquid phase method, a thermal spraying method, a baking method (sintering method), or the like. There are two or more types.

Since the coating 12C is formed on the surface of the negative electrode active material layer 12B, it covers the surface of the negative electrode active material layer 12B. However, the coating 12C may cover the entire surface of the negative electrode active material layer 12B, or may cover only a part of the surface of the negative electrode active material layer 12B. In the latter case, a plurality of coatings 12C may cover the surface of the negative electrode active material layer 12B at a plurality of locations separated from each other.

As described later, this film 12C is a film formed by charging and discharging the secondary battery for one or more cycles after the secondary battery is assembled. Therefore, in the negative electrode 12 of the secondary battery that has been charged and discharged for one or more cycles after assembly, a coating 12C is formed on the surface of the negative electrode active material layer 12B.

Specifically, "the negative electrode 12 in which the coating 12C is formed on the surface of the negative electrode active material layer 12B" means that the secondary battery has already been charged and discharged for one or more cycles after assembly, as described above. This means that the stabilization treatment described later has already been applied to the secondary battery. This stabilization treatment is a charge/discharge treatment for forming a film 12C containing an organic substance on the surface of the negative electrode active material layer 12B. In addition, as long as the secondary battery after assembly has been charged and discharged for one or more cycles, various conditions such as environmental temperature, number of charging and discharging (number of cycles), charging conditions, and discharging conditions are not particularly limited and can be set arbitrarily. It is.

The film 12C described here is a deposited film that mainly contains one or more types of decomposed products and reactants formed during charging and discharging, and the decomposed products and reactants are , a substance formed by decomposition and reaction of one or more of the carbon-containing components contained in the electrolytic solution described below. This "carbon-containing component" is a general term for components (compounds) contained in an electrolytic solution containing carbon as a constituent element. Therefore, the coating 12C contains an organic substance, that is, contains carbon as a constituent element.

Note that during charging and discharging, in order to form the coating 12C, the carbon-containing components may react with each other, the carbon-containing component may react with the non-carbon-containing component, or both may react. This "non-carbon-containing component" is a general term for components (compounds) contained in the electrolytic solution that do not contain carbon as a constituent element.

In order to confirm whether the film 12C containing organic matter is formed on the surface of the negative electrode active material layer 12B, X-ray photoelectron spectroscopy (XPS) is used to determine whether the surface of the negative electrode 12 (film 12C) is formed on the surface of the negative electrode active material layer 12B. 12C) is subjected to elemental analysis. In this elemental analysis, the analysis range is a portion from the surface of the coating 12C to a depth of 20 nm in the depth direction (that is, the thickness direction). For this reason, the thickness of the coating 12C is preferably greater than 20 nm, more specifically from 20 nm to 80 nm. This is because the surface of the negative electrode active material layer 12B is electrochemically protected by the coating 12C without intercalation and release of lithium being inhibited by the coating 12C during charging and discharging.

As a result of this elemental analysis using XPS, if carbon is detected, it means that the coating 12C is formed on the surface of the negative electrode active material layer 12B, that is, the secondary battery has already been charged and discharged for one or more cycles after assembly. This means that On the other hand, if carbon is not detected as a result of elemental analysis using XPS, it means that the coating 12C is not formed on the surface of the negative electrode active material layer 12B, that is, the secondary battery is not yet charged or discharged after assembly. It means that it is not

Note that when a commercially available secondary battery is used to check whether the coating 12C has been formed, it is considered that the coating 12C has already been formed. When a user uses a commercially available secondary battery, in order to ensure that the secondary battery can operate stably from the beginning, the commercially available secondary battery is manufactured at a stage (manufacturing stage) before being shipped to the market. This is because it is common that one or more cycles of charging and discharging treatment have already been performed in order to form the coating 12C in advance.

(Separator)
As shown in FIG. 2, the separator 13 is an insulating porous film interposed between the positive electrode 11 and the negative electrode 12, and prevents contact between the positive electrode 11 and negative electrode 12 while allowing lithium ions to flow through the separator 13. Let it pass. This separator 13 contains one or more of polymer compounds such as polytetrafluoroethylene, polypropylene, and polyethylene.

(electrolyte)
The electrolytic solution contains a solvent, an electrolyte salt, and an additive. This electrolytic solution is impregnated into each of the positive electrode 11, negative electrode 12, and separator 13.

(solvent)
The solvent contains one or more types of non-aqueous solvents (organic solvents), and the electrolytic solution containing the non-aqueous solvent is a so-called non-aqueous electrolytic solution. This nonaqueous solvent includes esters and ethers, and more specifically includes carbonate ester compounds, carboxylic ester compounds, and lactone compounds.

Carbonic ester compounds include cyclic carbonic esters and chain carbonic esters. Specific examples of cyclic carbonate esters include ethylene carbonate and propylene carbonate, and specific examples of chain carbonate esters include dimethyl carbonate, diethyl carbonate, and methylethyl carbonate. The carboxylic acid ester compound is a chain carboxylic acid ester. Specific examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, and trimethylethyl acetate. Lactone compounds include lactones. Specific examples of lactones include γ-butyrolactone and γ-valerolactone. However, the ethers may be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, etc. in addition to the above-mentioned lactone compounds.

Further, the non-aqueous solvent includes halogenated carbonate esters, phosphate esters, acid anhydrides, mononitrile compounds, isocyanate compounds, and the like. This is because the chemical stability of the electrolyte is improved. However, the fluorinated cyclic carbonate esters described below are excluded from the halogenated carbonate esters described here, and the sulfur-containing cyclic compounds described below are excluded from the acid anhydrides described here.

The halogenated carbonate esters include fluorinated chain carbonate esters. Specific examples of fluorinated chain carbonate esters include fluoromethylmethyl carbonate, bis(fluoromethyl) carbonate, and difluoromethylmethyl carbonate. Specific examples of phosphoric acid esters include trimethyl phosphate and triethyl phosphate. Acid anhydrides include cyclic dicarboxylic acid anhydrides. Specific examples of cyclic dicarboxylic anhydrides include succinic anhydride, glutaric anhydride, and maleic anhydride. Specific examples of mononitrile compounds include acetonitrile and acrylonitrile. A specific example of the isocyanate compound is hexamethylene diisocyanate.

Among these, it is preferable that the solvent contains a chain carboxylic acid ester. This is because the coating 12C is more likely to be stably formed using charge/discharge reactions. The content of the chain carboxylic acid ester in the solvent is not particularly limited, but is preferably 10% to 50% by weight. This is because the coating 12C is easily formed in a sufficiently stable manner.

(electrolyte salt)
The electrolyte salt is one or more types of light metal salts such as lithium salts. However, the oxalate metal salt described later is excluded from the electrolyte salt (lithium salt) described here.

Specific examples of lithium salts include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), and lithium bis(fluorosulfonyl)imide (LiN). (FSO ₂ ) ₂ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ₃ SO ₂ ) ₂ ), and lithium tris(trifluoromethanesulfonyl)methide (LiC(CF ₃ SO ₂ ) ₃ ).

The content of the electrolyte salt is not particularly limited, but is 0.3 mol/kg to 3.0 mol/kg relative to the solvent. This is because high ionic conductivity can be obtained.

(Additive)
The additive contains four types of compounds (sulfur-containing cyclic compound, fluorinated cyclic carbonate, unsaturated cyclic carbonate, and polynitrile chain compound) that participate in the formation of the coating 12C.

(Sulfur-containing cyclic compound)
As mentioned above, "sulfur-containing cyclic compound" is a general term for cyclic compounds containing one or more sulfur as a constituent element. In this sulfur-containing cyclic compound, one or more side chains may be bonded to the ring. In addition, the number of types of sulfur-containing cyclic compounds may be one, or two or more types.

However, the sulfur-containing cyclic compound contains one or more sulfur as an element constituting the ring. Therefore, compounds whose side chain contains sulfur as a constituent element but whose ring does not contain sulfur as a constituent element are excluded from the sulfur-containing cyclic compounds described herein. Note that as long as the ring contains sulfur as a constituent element, the side chain may contain sulfur as a constituent element, or the side chain does not need to contain sulfur as a constituent element.

The type of sulfur-containing cyclic compound is not particularly limited, but specific examples include cyclic sulfonic acid esters, cyclic disulfonic acid anhydrides, and cyclic carboxylic acid sulfonic anhydrides. Specific examples of cyclic sulfonic acid esters include propane sultone (1,3-propane sultone) and propene sultone (1-propene 1,3-sultone). Specific examples of the cyclic disulfonic anhydride include cyclic ethanedisulfonic anhydride and cyclic propanedisulfonic anhydride. Specific examples of the cyclic carboxylic acid sulfonic anhydride include sulfobenzoic anhydride, sulfopropionic anhydride, and sulfobutyric anhydride.

(Fluorinated cyclic carbonate)
As mentioned above, the "fluorinated cyclic carbonate ester" is a general term for cyclic carbonate esters containing one or more fluorine as a constituent element. That is, the fluorinated cyclic carbonate is a compound in which one or more hydrogen groups in the cyclic carbonate are substituted with a fluorine group. In addition, the number of types of fluorinated cyclic carbonate may be only one type, or two or more types may be used.

Specific examples of fluorinated cyclic carbonate esters include monofluoroethylene carbonate (4-fluoro-1,3-dioxolan-2-one) and difluoroethylene carbonate (4,5-fluoro-1,3-dioxolan-2-one). etc.

(Unsaturated cyclic carbonate)
As mentioned above, "unsaturated cyclic carbonate ester" is a general term for cyclic carbonate esters containing one or more unsaturated bonds (carbon-carbon double bonds). This carbon-carbon double bond may be contained in the ring or in one or more side chains bonded to the ring. In addition, the number of types of unsaturated cyclic carbonate may be only one type, or two or more types may be used.

Specific examples of unsaturated cyclic carbonates include vinylene carbonate (1,3-dioxol-2-one), vinylethylene carbonate (4-vinyl-1,3-dioxolan-2-one), and methyleneethylene carbonate (4-methylene -1,3-dioxolan-2-one).

(Multi-nitrile chain compound)
As mentioned above, the "multinitrile chain compound" is a general term for chain compounds containing two or more cyano groups. This multinitrile chain compound may contain two cyano groups, or may contain three or more cyano groups. In addition, the number of types of multinitrile chain compounds may be only one type, or two or more types.

The type of multinitrile chain compound is not particularly limited, but specifically includes a dinitrile chain compound containing two cyano groups and a trinitrile chain compound containing three cyano groups. Specific examples of dinitrile chain compounds include malononitrile, succinonitrile, glutaronitrile, adiponitrile, and sebaconitrile. A specific example of the trinitrile chain compound is 1,3,6-hexanetricarbonitrile.

Here, the reason why the electrolytic solution contains four types of compounds as additives (sulfur-containing cyclic compound, fluorinated cyclic carbonate, unsaturated cyclic carbonate, and polynitrile chain compound) is due to the three physical property conditions described below. This is because, when the conditions are satisfied, the electrical resistance of the secondary battery is less likely to increase. Note that details of the reason why the electrical resistance of the secondary battery is difficult to increase will be described later.

(Additive content)
The content of the sulfur-containing cyclic compound in the electrolytic solution is not particularly limited, but is preferably 0.5% to 1.2% by weight. The content of the fluorinated cyclic carbonate in the electrolytic solution is not particularly limited, but is preferably 1.2% by weight to 4.0% by weight. The content of the unsaturated cyclic carbonate in the electrolytic solution is not particularly limited, but is preferably 0.1% to 0.5% by weight. The content of the polynitrile chain compound in the electrolytic solution is not particularly limited, but is preferably 3.0% to 7.0% by weight. This is because it becomes difficult for the electrical resistance of the secondary battery to increase sufficiently.

Note that when the additive contains two or more types of sulfur-containing cyclic compounds, the content of the sulfur-containing cyclic compounds described above is the sum of the contents of each sulfur-containing cyclic compound. This fact that the content is the sum of two or more types of contents also applies to the respective contents of the fluorinated cyclic carbonate, the unsaturated cyclic carbonate, and the polynitrile chain compound.

In addition, the content of sulfur-containing cyclic compounds can be determined by collecting the electrolyte from the completed secondary battery and performing gas chromatography-mass spectrometry (GC/MS) as described below. It is measured by mass spectrometry of the electrolyte using The fact that the content is measured by mass spectrometry of the electrolyte using GC/MS is the same for the respective contents of fluorinated cyclic carbonate, unsaturated cyclic carbonate, and polynitrile chain compounds. be.

(Cyclic ether compound)
In addition, the additive may further contain any one type or two or more types of cyclic ether compounds. This is because the electrical resistance of the secondary battery is less likely to increase. This "cyclic ether compound" is a general term for cyclic compounds containing oxygen as a constituent element, and in the cyclic ether compound, one or more side chains may be bonded to the ring. However, the cyclic ether compound contains one or more oxygens as elements constituting the ring. Therefore, compounds whose side chains contain oxygen as a constituent element but whose rings do not contain oxygen as a constituent element are excluded from the cyclic ether compounds described herein.

Specific examples of the cyclic ether compound include tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydropyran, 1,3-dioxane and 1,4-dioxane.

The content of the cyclic ether compound in the electrolytic solution is not particularly limited, but is preferably 0.5% by weight to 1.0% by weight. This is because it becomes difficult for the electrical resistance of the secondary battery to increase sufficiently.

(oxalate metal salt)
Moreover, the additive may further contain any one type or two or more types of oxalic acid metal salts. This is because the electrical resistance of the secondary battery is less likely to increase. This "oxalate metal salt" is a general term for metal salts containing one or more oxalate groups (-O-C(=O)-C(=O)-O-), and the oxalate metal salts contains positive ions (cations) made of metal ions and negative ions (anions) containing one or more oxalate groups.

The type of metal ion that is a cation is not particularly limited, but specifically, it is a light metal ion such as an alkali metal ion, an alkaline earth metal ion, and an aluminum ion. Specific examples of alkali metal ions include lithium ions, sodium ions, and potassium ions. Specific examples of alkaline earth metal ions include beryllium ions, magnesium ions, and calcium ions.

The anion contains a central atom to which one or more oxalate groups are attached. The type of this central atom is not particularly limited, but includes boron atom (B), phosphorus atom (P), aluminum atom (Al), and the like.

Specific examples of oxalate metal salts include lithium difluorooxalatoborate, lithium bis(oxalato)borate, and lithium difluorobis(oxalato)phosphate.

The content of the oxalate metal salt in the electrolytic solution is not particularly limited, but is preferably 0.1% to 0.2% by weight. This is because it becomes difficult for the electrical resistance of the secondary battery to increase sufficiently.

[Positive lead and negative lead]
The positive electrode lead 14 is connected to the positive electrode 11 (positive electrode current collector 11A), and includes one or more types of conductive materials such as aluminum. The negative electrode lead 15 is connected to the negative electrode 12 (negative electrode current collector 12A), and includes one or more types of conductive materials such as copper, nickel, and stainless steel. The shape of each of the positive electrode lead 14 and the negative electrode lead 15 is either a thin plate shape or a mesh shape.

Here, each of the positive electrode lead 14 and the negative electrode lead 15 is led out in a common direction from the inside of the exterior film 20 to the outside, as shown in FIG. However, each of the positive electrode lead 14 and the negative electrode lead 15 may be led out in different directions.

Further, here, the number of positive electrode leads 14 is one. However, the number of positive electrode leads 14 is not particularly limited and may be two or more. In particular, when the number of positive electrode leads 14 is two or more, the electrical resistance of the secondary battery decreases. What has been described here regarding the number of positive electrode leads 14 also applies to the number of negative electrode leads 15, so the number of negative electrode leads 15 is not limited to one, but may be two or more.

<1-2. Physical properties>
In this secondary battery, as described above, when the electrolytic solution contains four types of compounds as additives (a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, and a polynitrile chain compound), In order to make it difficult for the electrical resistance of the secondary battery to increase, the three physical property conditions described below are simultaneously satisfied.

[First physical property condition]
The content of each of the sulfur-containing cyclic compound, fluorinated cyclic carbonate, unsaturated cyclic carbonate, and polynitrile chain compound in the electrolyte is measured by mass spectrometry of the electrolyte using GC/MS.

In this case, the content of each of the sulfur-containing cyclic compound, fluorinated cyclic carbonate, unsaturated cyclic carbonate, and polynitrile chain compound is determined as follows: content of polynitrile chain compound > content of fluorinated cyclic carbonate The following relationship (size relationship) is satisfied: >Content of sulfur-containing cyclic compound>Content of unsaturated cyclic carbonate ester.

That is, the content of the polynitrile chain compound is greater than the content of the fluorinated cyclic carbonate. Further, the content of the fluorinated cyclic carbonate is greater than the content of the sulfur-containing cyclic compound. Furthermore, the content of the sulfur-containing cyclic compound is greater than the content of the unsaturated cyclic carbonate.

[Second physical property conditions]
As mentioned above, when measuring the content of each of a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, and a polynitrile chain compound using GC/MS, the content of the sulfur-containing cyclic compound The total of the content, the content of the fluorinated cyclic carbonate, the content of the unsaturated cyclic carbonate, and the content of the polynitrile chain compound is 5.0% by weight to 11.0% by weight.

[Third physical property conditions]
When the elemental concentration of carbon (atomic %) is determined by elemental analysis of the surface of the negative electrode 12 (coating 12C) using XPS, the elemental concentration of carbon is 80 atomic % to 94 atomic %. However, in the elemental analysis using XPS, as described above, the analysis range is a portion from the surface of the coating 12C to a depth of 20 nm in the depth direction.

<1-3. Operation＞
When charging the secondary battery, lithium is released from the positive electrode 11, and at the same time, the lithium is inserted into the negative electrode 12 via the electrolyte. Furthermore, when the secondary battery is discharged, lithium is released from the negative electrode 12, and the lithium is inserted into the positive electrode 11 via the electrolyte. During these charging and discharging operations, lithium is intercalated and released in an ionic state.

<1-4. Manufacturing method＞
When manufacturing a secondary battery, the positive electrode 11 and negative electrode 12 are manufactured and an electrolyte is prepared according to the procedure described below, and then a secondary battery is manufactured using the positive electrode 11, negative electrode 12, and electrolyte. do.

[Preparation of positive electrode]
First, a paste-like positive electrode mixture slurry is prepared by adding a positive electrode mixture in which a positive electrode active material and a positive electrode binder are mixed together into an organic solvent. A positive electrode active material layer 11B is formed by applying a positive electrode mixture slurry to the substrate. Thereafter, the positive electrode active material layer 11B may be compression molded using a roll press machine or the like. In this case, the positive electrode active material layer 11B may be heated or compression molding may be repeated multiple times. As a result, the positive electrode active material layers 11B are formed on both sides of the positive electrode current collector 11A, so that the positive electrode 11 is manufactured.

[Preparation of negative electrode]
First, a paste-like negative electrode mixture slurry is prepared by adding a negative electrode mixture in which a negative electrode active material, a negative electrode binder, etc. are mixed together into an organic solvent, etc. A negative electrode active material layer 12B is formed by applying a negative electrode mixture slurry to the substrate. Thereafter, the negative electrode active material layer 12B may be compression molded using a roll press or the like, similar to the above-described manufacturing procedure for the positive electrode 11. Subsequently, as will be described later, after the secondary battery is assembled, the secondary battery is stabilized (charged and discharged). As a result, a coating 12C containing an organic substance is formed on the surface of the negative electrode active material layer 12B. Therefore, the negative electrode 12 is manufactured because the negative electrode active material layer 12B and the coating 12C are formed on both sides of the negative electrode current collector 12A.

[Preparation of electrolyte]
After the electrolyte salt is added to the solvent, additives (sulfur-containing cyclic compound, fluorinated cyclic carbonate, unsaturated cyclic carbonate, and polynitrile chain compound) are added to the solvent. As a result, the electrolyte salt and the additive are each dispersed or dissolved in the solvent, so that an electrolytic solution is prepared. In addition, when preparing the electrolytic solution, other additives (one or both of the cyclic ether compound and the oxalic acid metal salt) may be further added to the solvent.

[Assembling the secondary battery]
First, the positive electrode lead 14 is connected to the positive electrode 11 (positive electrode current collector 11A) using a welding method or the like, and the negative electrode lead 15 is connected to the negative electrode 12 (negative electrode current collector 12A) using a welding method or the like.

Subsequently, the positive electrode 11 and the negative electrode 12 are laminated with each other via the separator 13, and then the positive electrode 11, the negative electrode 12, and the separator 13 are wound to produce a wound body. This wound body has a configuration similar to that of the battery element 10, except that each of the positive electrode 11, the negative electrode 12, and the separator 13 is not impregnated with an electrolytic solution. Subsequently, the rolled body is pressed using a press or the like to shape the rolled body into a flat shape.

Subsequently, after storing the rolled body inside the recessed portion 20U, the exterior films 20 are folded up so that the exterior films 20 face each other. Next, by using a heat fusion method or the like to fuse the outer peripheral edges of two sides of the exterior films 20 (fusion layers) facing each other to each other, the inside of the bag-shaped exterior film 20 is sealed. Store the rolled body.

Finally, after injecting the electrolyte into the inside of the bag-shaped exterior film 20, the outer peripheral edges of the remaining one side of the exterior film 20 (fusion layer) are fused together using a heat fusion method or the like. Let them wear it. In this case, an adhesive film 21 is inserted between the exterior film 20 and the positive electrode lead 14, and an adhesive film 22 is inserted between the exterior film 20 and the negative electrode lead 15. As a result, the wound body is impregnated with the electrolytic solution, so that the battery element 10 is manufactured. Therefore, since the battery element 10 is sealed inside the bag-shaped exterior film 20, a secondary battery is assembled.

[Stabilization of secondary batteries]
Charge and discharge the assembled secondary battery. Various conditions such as environmental temperature, number of times of charging and discharging (number of cycles), charging conditions, and discharging conditions can be arbitrarily set. Thereby, as described above, the film 12C containing organic matter is formed on the surface of the negative electrode active material layer 12B, so that the state of the secondary battery is electrochemically stabilized. Therefore, a secondary battery using the exterior film 20, that is, a laminate film type secondary battery is completed.

<1-5. Action and effect>
According to this secondary battery, the electrolytic solution contains additives (a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, and a polynitrile chain compound), and the negative electrode 12 has a coating 12C containing an organic substance. is formed on the surface of the negative electrode active material layer 12B, and the three physical property conditions described above regarding the mass spectrometry of the electrolyte using GC/MS and the elemental analysis of the coating 12C using XPS are simultaneously satisfied. Therefore, excellent electrical resistance characteristics can be obtained for the reasons explained below.

When the electrolyte solvent contains a carbonate ester compound, etc., the dissociation properties of the electrolyte salt and the mobility of ions improve, resulting in excellent cycle characteristics, but on the other hand, when charging and discharging are repeated, As the solvent decomposition reaction progresses, electrical resistance increases. This is because the film formed on the surface of the negative electrode 12 due to the decomposition reaction of the solvent has high electrical resistance, so if the film continues to be formed as the decomposition reaction progresses, the film will This is because the electrical resistance of the negative electrode 12 and the like increases due to its presence.

In order to suppress the increase in electrical resistance caused by the decomposition reaction of the solvent, it is conceivable to increase the amount of electrolyte solution. However, simply increasing the amount of electrolyte will result in the presence of an excessive amount of electrolyte inside the secondary battery, which will not only damage the appearance of the secondary battery but also reduce its energy density. Resulting in.

Therefore, instead of increasing the amount of electrolytic solution, it may be possible to add to the electrolytic solution an additive that has the property of being preferentially more easily decomposed than the solvent. When this additive is consumed to form a film containing organic matter on the surface of the negative electrode 12 during the stabilization process (charging and discharging process) of the secondary battery, it remains in the electrolyte even after the charging and discharging process. When repeated, it is consumed to continuously form a film on the surface of the negative electrode 12. This suppresses the decomposition reaction of the solvent even if charging and discharging are repeated, making it difficult for electrical resistance to increase.

In this case, especially when two or more different additives are used in combination, the decomposition reaction of the solvent tends to be continuously suppressed. This is because the degradability (ease of decomposition) of additives differs depending on the type of additive, so by using two or more types of additives with different degradability, the decomposition reaction of each additive This is because the periods required from the start to the completion can be shifted from each other. As a result, the time when all the additives are completely decomposed, that is, the time when the additives are depleted due to the decomposition reaction, is delayed, and the time when the solvent starts to decompose is also delayed. Therefore, the timing at which the electrical resistance starts to increase in the secondary battery is delayed, so that an advantage is obtained in that the increase in electrical resistance can be suppressed.

However, in order to obtain the above-mentioned advantages, it is not enough to simply use two or more types of additives together; in reality, it is necessary to accurately understand the degradability (for example, decomposition rate, etc.) of each additive. Depending on the combination of two or more types of additives, it is necessary to optimize the content of those additives (the content of each additive and the sum of their contents). This is because, even if two or more types of additives are used together, if the content of each additive and the sum of those contents are not optimized, the time when the decomposition reaction of each additive will start or complete will be adjusted to each other. This is because the decomposition reactions of each additive may be completed almost simultaneously. In particular, when charging and discharging are repeated, if the decomposition reaction of all additives is completed during the initial charging and discharging, the solvent will begin to decompose from that initial charging and discharging, so two or more types of additives may be used in the first place. There is no point in using them together.

Based on these technical backgrounds, in the secondary battery of this embodiment, the electrolyte contains four types of compounds as additives (sulfur-containing cyclic compound, fluorinated cyclic carbonate ester, unsaturated cyclic carbonate), as described above. carbonate ester and polynitrile chain compound), the three physical property conditions related to the analysis results (GC/MS and is formed.

In this case, four types of compounds with mutually different degradability are used as additives, and the content of each additive (the size relationship between the contents of each additive) and the sum of these contents are optimized. be done. As a result, the times at which the decomposition reactions of the respective additives start or complete are set to be appropriately shifted from each other, so that the time at which the solvent begins to decompose is sufficiently delayed. That is, the time when the electrical resistance starts to increase due to the decomposition reaction of the solvent is sufficiently delayed, so that the increase in the electrical resistance is effectively suppressed. Therefore, since electrical resistance is less likely to increase, excellent electrical resistance characteristics can be obtained.

In particular, the content of sulfur-containing cyclic compounds is 0.5% to 1.2% by weight, the content of fluorinated cyclic carbonates is 1.2% to 4.0% by weight, and the content of unsaturated cyclic carbonates is When the amount is 0.1% to 0.5% by weight and the content of the multinitrile chain compound is 3.0% to 7.0% by weight, the above three physical property conditions are easily satisfied at the same time. . Therefore, it becomes difficult for the electrical resistance of the secondary battery to increase sufficiently, so that higher effects can be obtained.

In addition, if the electrolytic solution further contains a cyclic ether compound as an additive and the content of the cyclic ether compound in the electrolytic solution is 0.5% to 1.0% by weight, the electrical resistance of the secondary battery is less likely to increase, so higher effects can be obtained.

In addition, if the electrolytic solution further contains a metal oxalate as an additive, and the content of the metal oxalate in the electrolytic solution is 0.1% to 0.2% by weight, the electrical resistance of the secondary battery will increase. is less likely to increase, so higher effects can be obtained.

Further, if the solvent of the electrolytic solution contains a chain carboxylic ester and the content of the chain carboxylic ester in the solvent is 10% to 50% by weight, the coating 12C can be formed sufficiently stably. Since it is easier to use, higher effects can be obtained.

Further, if the secondary battery is a lithium ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing intercalation and desorption of lithium, so that higher effects can be obtained.

<2. Modified example>
Next, a modification of the secondary battery will be described. The configuration of the secondary battery described above can be modified as appropriate, as described below. However, the series of modifications described below may be combined with each other.

[Modification 1]
A separator 13, which is a porous membrane, was used. However, although not specifically illustrated here, a laminated separator including a polymer compound layer may be used instead of the separator 13 which is a porous membrane.

Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer disposed on one or both sides of the porous membrane. This is because the adhesion of the separator to each of the positive electrode 11 and the negative electrode 12 is improved, so that misalignment of the battery element 10 becomes less likely to occur. This makes it difficult for the secondary battery to swell even if a decomposition reaction of the electrolyte occurs. The polymer compound layer contains a polymer compound such as polyvinylidene fluoride that has excellent physical strength and is electrochemically stable.

Note that one or both of the porous membrane and the polymer compound layer may contain any one type or two or more types of the plurality of insulating particles. This is because the plurality of insulating particles radiate heat when the secondary battery generates heat, improving the safety (heat resistance) of the secondary battery. Insulating particles include inorganic particles and resin particles. Specific examples of inorganic particles are particles of aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide and zirconium oxide. Specific examples of resin particles are particles of acrylic resin and styrene resin.

When producing a laminated separator, a precursor solution containing a polymer compound, an organic solvent, etc. is prepared, and then the precursor solution is applied to one or both sides of the porous membrane. In this case, a plurality of insulating particles may be added to the precursor solution as necessary.

Even when this laminated separator is used, the same effect can be obtained because lithium ions can move between the positive electrode 11 and the negative electrode 12.

[Modification 2]
An electrolytic solution, which is a liquid electrolyte, was used. However, although not specifically illustrated here, an electrolyte layer that is a gel-like electrolyte may be used instead of the electrolyte.

In the battery element 10 using an electrolyte layer, a positive electrode 11 and a negative electrode 12 are stacked on each other via a separator 13 and an electrolyte layer, and then the positive electrode 11, negative electrode 12, separator 13, and electrolyte layer are wound. This electrolyte layer is interposed between the positive electrode 11 and the separator 13 and also between the negative electrode 12 and the separator 13.

Specifically, the electrolyte layer contains a polymer compound together with an electrolyte solution, and the electrolyte solution is held by the polymer compound in the electrolyte layer. This is because leakage is prevented. The structure of the electrolytic solution is as described above. The polymer compound includes polyvinylidene fluoride and the like. When forming an electrolyte layer, a precursor solution containing an electrolytic solution, a polymer compound, an organic solvent, etc. is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 11 and the negative electrode 12.

Even when this electrolyte layer is used, the same effect can be obtained because lithium ions can move between the positive electrode 11 and the negative electrode 12 via the electrolyte layer.

<3. Applications of secondary batteries>
Next, the uses (application examples) of the above-described secondary battery will be explained.

The main uses of secondary batteries are machines, equipment, appliances, devices, and systems (aggregates of multiple devices, etc.) in which secondary batteries can be used as power sources for driving or power storage sources for power storage. etc., there are no particular limitations. The secondary battery used as a power source may be a main power source or an auxiliary power source. The main power source is a power source that is used preferentially, regardless of the presence or absence of other power sources. The auxiliary power source may be a power source used in place of the main power source, or may be a power source that can be switched from the main power source as needed. When using a secondary battery as an auxiliary power source, the type of main power source is not limited to the secondary battery.

Specific examples of uses of secondary batteries are as follows. Electronic devices (including portable electronic devices) such as video cameras, digital still cameras, mobile phones, notebook computers, cordless telephones, headphone stereos, portable radios, portable televisions, and portable information terminals. These are portable household appliances such as electric shavers. Backup power supplies and storage devices such as memory cards. Power tools such as power drills and power saws. A battery pack that is installed in notebook computers and other devices as a removable power source. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric vehicles (including hybrid vehicles). This is a power storage system such as a home battery system that stores power in case of an emergency. Note that the battery structure of the secondary battery may be the above-described laminate film type or cylindrical type, or may be any other battery structure other than these. Further, a plurality of secondary batteries may be used as a battery pack, a battery module, and the like.

Among these, battery packs and battery modules are effectively applied to relatively large devices such as electric vehicles, power storage systems, and power tools. The battery pack may be a single cell or an assembled battery, as will be described later. An electric vehicle is a vehicle that operates (travels) using a secondary battery as a driving power source, and as described above, may be a vehicle (such as a hybrid vehicle) that also includes a driving source other than the secondary battery. A power storage system is a system that uses a secondary battery as a power storage source. In a home power storage system, power is stored in a secondary battery, which is a power storage source, so that the power can be used to use home electrical appliances and the like.

Here, an example of the application of the secondary battery will be specifically described. The configuration of the application example described below is just an example and can be modified as appropriate.

FIG. 3 shows the block configuration of the battery pack. The battery pack described here is a simple battery pack (so-called soft pack) using one secondary battery, and is installed in electronic devices such as smartphones.

This battery pack includes a power source 41 and a circuit board 42, as shown in FIG. This circuit board 42 is connected to a power source 41 and includes a positive terminal 43, a negative terminal 44, and a temperature detection terminal 45. The temperature detection terminal 45 described here is a so-called T terminal.

Power source 41 includes one secondary battery. In this secondary battery, the positive electrode lead is connected to the positive electrode terminal 43, and the negative electrode lead is connected to the negative electrode terminal 44. This power source 41 can be connected to the outside via the positive terminal 43 and the negative terminal 44, and therefore can be charged and discharged via the positive terminal 43 and the negative terminal 44. The circuit board 42 includes a control section 46 , a switch 47 , a heat sensitive resistance element (Positive Temperature Coefficient (PTC) element) 48 , and a temperature detection section 49 . However, the PTC element 48 may be omitted.

The control unit 46 includes a central processing unit (CPU), a memory, and the like, and controls the operation of the entire battery pack. This control unit 46 detects and controls the usage state of the power source 41 as necessary.

Note that when the battery voltage of the power source 41 (secondary battery) reaches the overcharge detection voltage or overdischarge detection voltage, the control unit 46 disconnects the switch 47 so that no charging current flows through the current path of the power source 41. Do it like this. Further, when a large current flows during charging or discharging, the control unit 46 cuts off the charging current by turning off the switch 47. The overcharge detection voltage and overdischarge detection voltage are not particularly limited. For example, the overcharge detection voltage is 4.2V±0.05V, and the overdischarge detection voltage is 2.4V±0.1V.

The switch 47 includes a charging control switch, a discharging control switch, a charging diode, a discharging diode, and the like, and switches whether or not the power source 41 is connected to an external device in accordance with an instruction from the control unit 46. The switch 47 includes a metal-oxide-semiconductor field-effect transistor (MOSFET), etc., and the charging/discharging current is detected based on the ON resistance of the switch 47. Ru.

The temperature detection section 49 includes a temperature detection element such as a thermistor, and measures the temperature of the power supply 41 using the temperature detection terminal 45 and outputs the temperature measurement result to the control section 46 . The temperature measurement result measured by the temperature detection unit 49 is used when the control unit 46 performs charge/discharge control during abnormal heat generation, and when the control unit 46 performs correction processing when calculating the remaining capacity.

An example of the present technology will be described.

(Experiment Examples 1 to 44)
As explained below, after producing the laminate film type secondary battery (lithium ion secondary battery) shown in FIGS. 1 and 2, the electrical resistance characteristics of the secondary battery were evaluated.

[Preparation of secondary battery]
A secondary battery was produced according to the following procedure.

(Preparation of positive electrode)
First, a positive electrode mixture was prepared by mixing 91 parts by mass of a positive electrode active material (LiCoO ₂ ), 3 parts by mass of a positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of a positive electrode conductive agent (graphite). . Subsequently, the positive electrode mixture was added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like positive electrode mixture slurry. Next, a positive electrode mixture slurry is applied to both sides of the positive electrode current collector 11A (a strip-shaped aluminum foil having a thickness of 12 μm) using a coating device, and then the positive electrode mixture slurry is dried to form a positive electrode active material. A material layer 11B was formed. Finally, the positive electrode active material layer 11B was compression molded using a roll press machine. As a result, the positive electrode active material layers 11B were formed on both sides of the positive electrode current collector 11A, so that the positive electrode 11 was manufactured.

(Preparation of negative electrode)
First, 93 parts by mass of a negative electrode active material (artificial graphite) and 7 parts by mass of a negative electrode binder (polyvinylidene fluoride) were mixed to prepare a negative electrode mixture. Subsequently, the negative electrode mixture was added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like negative electrode mixture slurry. Next, a coating device is used to apply a negative electrode mixture slurry to both sides of the negative electrode current collector 12A (a strip-shaped copper foil having a thickness of 15 μm), and then the negative electrode mixture slurry is dried to form a negative electrode active material. A material layer 12B was formed. Subsequently, the negative electrode active material layer 12B was compression molded using a roll press machine. Finally, a coating 12C containing an organic substance was formed on the surface of the negative electrode active material layer 12B by performing a secondary battery stabilization process to be described later. As a result, the negative electrode active material layer 12B and the coating 12C were formed on both sides of the negative electrode current collector 12A, so that the negative electrode 12 was manufactured.

(Preparation of electrolyte)
First, an electrolyte salt (lithium hexafluorophosphate) was added to the solvent, and then the solvent was stirred. As solvents, ethylene carbonate (EC) and propylene carbonate (PC), which are cyclic carbonates, diethyl carbonate (DEC), which is a chain carbonate, and propyl propionate (PrPr), which is a chain carboxylic ester, are used. Using. The mixing ratio (volume ratio) of the solvent was EC:PC:DEC:PrPr=10:5:15:25. The content of the electrolyte salt was 1 mol/kg relative to the solvent.

Finally, after adding additives (sulfur-containing cyclic compound, fluorinated cyclic carbonate, unsaturated cyclic carbonate, and polynitrile chain compound) to the solvent, the solvent was stirred. As a result, the electrolyte salt and the additive were each dispersed or dissolved in the solvent, so that an electrolytic solution was prepared.

As the sulfur-containing cyclic compound, propane sultone (PS), which is a cyclic sulfonic acid ester, and cyclic propane disulfonic anhydride (PSAH), which is a cyclic disulfonic acid anhydride, were used. In addition, when using a cyclic sulfonic acid ester and a cyclic disulfonic acid anhydride together, the mixing ratio (weight ratio) was 1:1.

As the fluorinated cyclic carbonate esters, monofluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC) were used.

As the unsaturated cyclic carbonate esters, vinylene carbonate (VC) and vinylethylene carbonate (VEC) were used.

As the multinitrile chain compound, succinonitrile (SN) and adiponitrile (ADN), which are dinitrile chain compounds, and 1,3,6-hexanetricarbonitrile (HTN), which is a trinitrile chain compound, were used. . In addition, when a dinitrile chain compound and a trinitrile chain compound were used together, the mixing ratio (weight ratio) was 1:1.

(Assembling secondary battery)
First, the positive electrode lead 14 made of aluminum was welded to the positive electrode current collector 11A, and the negative electrode lead 15 made of copper was welded to the negative electrode current collector 12A. Subsequently, the positive electrode 11 and the negative electrode 12 are laminated with each other via the separator 13 (a microporous polyethylene film having a thickness of 15 μm), and then the positive electrode 11, the negative electrode 12, and the separator 13 are wound. A rotating body was created.

Next, after folding the exterior film 20 so as to sandwich the rolled body housed in the recessed portion 20U, the outer peripheral edges of two sides of the exterior film 20 are heat-sealed to each other to form a bag-like shape. The rolled body was housed inside the exterior film 20 of. This exterior film 20 includes a fusion layer (polypropylene film with a thickness of 30 μm), a metal layer (aluminum foil with a thickness of 40 μm), and a surface protection layer (a nylon film with a thickness of 25 μm). An aluminum laminate film was used in which the film was laminated in this order from the inside.

Subsequently, an electrolytic solution was injected into the inside of the bag-shaped exterior film 20, and then the outer peripheral edges of the remaining one side of the exterior film 20 were heat-sealed together in a reduced pressure environment. In this case, an adhesive film 21 (a polypropylene film with a thickness of 5 μm) is inserted between the exterior film 20 and the positive electrode lead 14, and an adhesive film 22 (with a thickness of 5 μm) is inserted between the exterior film 20 and the negative electrode lead 15. A polypropylene film (with a diameter of 5 μm) was inserted. As a result, the wound body was impregnated with the electrolytic solution, so that the battery element 10 was manufactured. Therefore, since the battery element 10 was sealed inside the exterior film 20, a secondary battery was assembled.

(Stabilization of secondary batteries)
The secondary battery was charged and discharged for one cycle in a constant temperature bath (temperature = 70°C). At the time of charging, constant current charging was performed at a current of 0.1C until the voltage reached 4.2V, and then constant voltage charging was performed at the voltage of 4.2V until the current reached 0.05C. During discharge, constant current discharge was performed at a current of 0.1C until the voltage reached 2.5V. 0.1C is a current value that completely discharges the battery capacity (theoretical capacity) in 10 hours, and 0.05C is a current value that completely discharges the above-mentioned battery capacity in 20 hours. Thereby, as described above, the film 12C containing an organic substance was formed on the surface of the negative electrode active material layer 12B, so that a laminate film type secondary battery was completed.

After collecting the electrolyte from the completed secondary battery, the electrolyte was subjected to mass spectrometry using GC/MS. As a result, as shown in Tables 1 to 3, the respective contents (wt%) of sulfur-containing cyclic compounds, fluorinated cyclic carbonates, unsaturated cyclic carbonates, and polynitrile chain compounds in the electrolyte are Measured.

In addition, in the "Total sum (wt%)" column of each of Tables 1 to 3, the content of sulfur-containing cyclic compounds, the content of fluorinated cyclic carbonates, the content of unsaturated cyclic carbonates, and the content of polynitrile The total sum with the content of chain compounds is shown.

In the "size relationship" column of each of Tables 1 to 3, content of sulfur-containing cyclic compounds > content of fluorinated cyclic carbonates > content of unsaturated cyclic carbonates > content of polynitrile chain compounds It shows whether the relationship of quantity (size relationship) is established or not. Specifically, "established" indicates that the above-described magnitude relationship is established, and "not established" indicates that the above-described magnitude relationship is not established.

Further, after collecting the negative electrode 12 from the completed secondary battery, the coating 12C was subjected to elemental analysis using XPS. As a result, the elemental concentration (atomic %) of carbon was measured as shown in Tables 1 to 3.

[Evaluation of electrical resistance characteristics]
When the electrical resistance characteristics of the secondary battery were evaluated, the results shown in Tables 1 to 3 were obtained.

When examining the electrical resistance characteristics, first, the electrical resistance (initial resistance (mΩ)) of the secondary battery was measured using a battery tester in a normal temperature environment (temperature = 23° C.). Subsequently, the secondary battery was charged and discharged for 400 cycles in a high temperature environment (temperature = 45°C), and then the electrical resistance (resistance after high temperature cycle (mΩ)) of the secondary battery was measured again using a battery tester. The charging and discharging conditions were the same as those for the stabilization treatment of the secondary battery described above. Finally, resistance increase rate (%)=[(resistance after high temperature cycle−initial resistance)/initial resistance]×100 was calculated. Note that each of Tables 1 to 3 shows the initial resistance along with the resistance increase rate.

[Consideration]
As shown in Tables 1 to 3, the electrolytic solution contains additives (a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, and a polynitrile chain compound), and the coating 12C also contains an organic substance. The electrical resistance characteristics of the secondary battery formed on the surface of the negative electrode active material layer 12B vary greatly depending on the mass spectrometry results of the electrolyte using GC/MS and the elemental analysis results of the coating 12C using XPS. did.

Specifically, when the following three physical property conditions are met at the same time (Experimental Examples 1 to 17), the results are compared with cases where the three physical property conditions are not met at the same time (Experimental Examples 18 to 44). As a result, the resistance increase rate was significantly reduced.

- First physical property condition The following magnitude relationship is satisfied: content of polynitrile chain compound > content of fluorinated cyclic carbonate > content of sulfur-containing cyclic compound > content of unsaturated cyclic carbonate.
・Second physical property condition The sum of the content of the sulfur-containing cyclic compound, the content of the fluorinated cyclic carbonate, the content of the unsaturated cyclic carbonate, and the content of the dinitrile chain compound is 5.0% by weight to 11% by weight. .0% by weight.
- Third physical property condition The elemental concentration of carbon is 80 atomic % to 94 atomic %.

In particular, when the three physical property conditions are satisfied at the same time, the content of the sulfur-containing cyclic compound is 0.5% to 1.2% by weight, and the content of the fluorinated cyclic carbonate is 1.2% by weight. -4.0% by weight, the content of unsaturated cyclic carbonate is 0.1% - 0.5% by weight, and the content of polynitrile chain compounds is 3.0% - 7.0% by weight. , the resistance increase rate was sufficiently reduced.

(Experiment examples 45 to 55)
As shown in Table 4, a secondary battery was prepared using the same procedure except that the composition of the solvent was changed and other additives (cyclic ether compound and metal oxalate) were added to the electrolyte. At the same time, the electrical resistance characteristics were evaluated.

When changing the composition of the solvent, as shown in Table 4, the content (wt%) of the chain carboxylic ester (PrPr) in the solvent was changed, and the chain carboxylic ester was newly added. Ethyl propionate (PrEt) was used. In addition, when changing the content of the chain carboxylic ester, change the content of the chain carboxylic ester so that the sum of the chain carbonate ester content and the chain carboxylic ester content is constant. The content of the chain carbonate ester was increased or decreased in accordance with the increase or decrease in the content.

As the cyclic ether compound, 1,4-dioxane (DOX) was used, and as the oxalate metal salt, lithium difluorooxalatoborate (LiFOB) was used. The respective contents (wt%) of the cyclic ether compound and the oxalate metal salt in the electrolytic solution are as shown in Table 4.

As shown in Table 4, even if the composition of the solvent is changed and other additives are added to the electrolyte, if the three physical property conditions are simultaneously satisfied (Experimental Examples 45 to 55), The resistance increase rate has been significantly reduced.

In particular, when the electrolytic solution further contained a cyclic ether compound and the content of the cyclic ether compound in the electrolytic solution was 0.5% to 1.0% by weight, the resistance increase rate was further reduced. At the same time, the initial resistance also decreased in some cases. In addition, when the electrolytic solution further contains a metal oxalate salt and the content of the metal oxalate in the electrolytic solution is 0.1% to 0.2% by weight, the rate of increase in resistance is sufficiently reduced. However, the initial resistance was lower.

In addition, if the solvent of the electrolytic solution contains a chain carboxylic ester, if the content of the chain carboxylic ester in the solvent is 10% to 50% by weight, the initial resistance will be sufficient. While decreasing, the rate of increase in resistance also decreased sufficiently.

[summary]
From the results shown in Tables 1 to 4, it is clear that the electrolytic solution contains a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, and other nitrile chain compounds, and that the coating 12C containing organic matter is active in the negative electrode. In the secondary battery formed on the surface of the material layer 12B, the three physical property conditions described above regarding the mass spectrometry of the electrolyte using GC/MS and the elemental analysis of the coating 12C using XPS are simultaneously satisfied. , the resistance increase rate was significantly reduced. Therefore, excellent electrical resistance characteristics were obtained in the secondary battery.

Although the present technology has been described above with reference to one embodiment and an example, the configuration of the present technology is not limited to the configuration described in the one embodiment and example, and can be variously modified.

Although the case where the battery structure of the secondary battery is a laminate film type has been described, the battery structure is not particularly limited. Specifically, the battery structure may be cylindrical, square, coin-shaped, button-shaped, or the like.

Moreover, although the case where the element structure of the battery element is a wound type has been described, the element structure is not particularly limited. Specifically, the element structure may be a stacked type in which the electrodes (positive electrode and negative electrode) are stacked, a 99-fold type in which the electrodes (positive electrode and negative electrode) are folded in a zigzag pattern, or the like.

Furthermore, although the case where the electrode reactant is lithium has been described, the electrode reactant is not particularly limited. Specifically, the electrode reactants may be other alkali metals, such as sodium and potassium, or alkaline earth metals, such as beryllium, magnesium, and calcium, as described above. In addition, the electrode reactant may be other light metals such as aluminum.

Since the effects described in this specification are merely examples, the effects of the present technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with the present technology.

Claims (6)

a positive electrode;
a negative electrode in which a film containing an organic substance is formed on the surface of a negative electrode active material layer;
An electrolytic solution containing a sulfur-containing cyclic compound, a fluorinated cyclic carbonate, an unsaturated cyclic carbonate, and a polynitrile chain compound,
When the electrolyte is mass analyzed using gas chromatography-mass spectrometry (GC/MS),
The content of each of the sulfur-containing cyclic compound, the fluorinated cyclic carbonate, the unsaturated cyclic carbonate, and the polynitrile chain compound in the electrolytic solution is such that the content of the polynitrile chain compound>the fluorine Satisfying the relationship: content of cyclic cyclic carbonate>content of the sulfur-containing cyclic compound>content of the unsaturated cyclic carbonate,
The total of the content of the sulfur-containing cyclic compound, the content of the fluorinated cyclic carbonate, the content of the unsaturated cyclic carbonate, and the content of the polynitrile chain compound is 5.0 weight. % or more and 11.0% by weight or less,
Elemental analysis of the coating from the surface to a depth of 20 nm using X-ray photoelectron spectroscopy (XPS) reveals that
The elemental concentration of carbon is 80 atomic % or more and 94 atomic % or less,
Secondary battery. The content of the sulfur-containing cyclic compound is 0.5% by weight or more and 1.2% by weight or less,
The content of the fluorinated cyclic carbonate is 1.2% by weight or more and 4.0% by weight or less,
The content of the unsaturated cyclic carbonate is 0.1% by weight or more and 0.5% by weight or less,
The content of the polynitrile chain compound is 3.0% by weight or more and 7.0% by weight or less,
The secondary battery according to claim 1. The electrolyte further includes a cyclic ether compound,
The content of the cyclic ether compound in the electrolytic solution is 0.5% by weight or more and 1.0% by weight or less,
The secondary battery according to claim 1 or 2. The electrolyte further includes an oxalate metal salt,
The content of the oxalate metal salt in the electrolytic solution is 0.1% by weight or more and 0.2% by weight or less,
The secondary battery according to any one of claims 1 to 3. The electrolyte solution includes a solvent and an electrolyte salt,
The solvent contains a chain carboxylic acid ester,
The content of the chain carboxylic acid ester in the solvent is 10% by weight or more and 50% by weight or less,
The secondary battery according to any one of claims 1 to 4. A lithium ion secondary battery,
The secondary battery according to any one of claims 1 to 5.

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